1. Field of the Invention
The present invention relates to fruit- and nut-harvesting equipment, and more particularly to apparatus for shaking fruit and nuts from the limbs of trees.
2. Description of the Related Art
Mechanical devices for harvesting fruit and nuts from trees typically operate by gripping the tree trunk and imparting thereto a controlled, low-frequency vibration or shaking action. This operation dislodges the fruit or nuts, which are then collected and transported.
A typical tree-shaking apparatus includes two opposing clamp members that engage the tree trunk, and which are themselves driven by dual oscillation or vibration units. In conventional systems these units operate substantially independently of one another; see, e.g., U.S. Pat. No. 3,338,040. This results in shaking of the tree in a number of different, random directions. Such action is undesirable, because some of these directions may be injurious to the tree; excessive transverse shear, for example, can strip tree bark and abrade the stem. Moreover, two randomly vibrating shakers can at times oppose one another, dissipating energy; or excessively reinforce one another, exerting undue compressive force on the tree.
Some efforts have been made to coordinate the action of the two vibration units, for example, by altering the moments of inertia of the rotators as they spin; see, e.g., U.S. Pat. Nos. 3,548,578 and 4,903,471. Even these improved devices, however, wrench the tree across a range of directions at once, risking damage to the root system. Apparatus for altering a rotator's moment of inertia can, in addition, be rather complex, as exemplified by the '471 patent.
Practitioners have also experimented with variation of the frequency of shaking. Every physical system possesses a unique, natural resonance frequency. Vibrating the system at its resonance frequency results in the maximum amplitude of displacement and, in the resent context, the most efficient removal of fruit or nuts. Thus, U.S. Pat. No. 3,650,099 discloses a limb shaker having a variable throttle that the user can adjust until the greatest displacement is observed. Such a fully manual approach is both cumbersome and imprecise, and is therefore poorly suited to commercial harvesting.